This invention relates to gamma cameras and more particularly to a novel and improved detector head for a gamma camera.
In nuclear medicine diagnostic procedures, radioactive isotopes are administered to patients. These isotopes tend to concentrate in certain organs and tumors.
After the isotope has been administered an image of the spatial distribution of the isotope is developed to produce an aid to medical diagnosis. For example, if a patient has a brain tumor and radioactive technetium has been administered, the technetium will tend to concentrate in the tumor and an image of the technetium will show the location and size of the tumor.
The preferred imaging device for most diagnostic nuclear medical imaging is often referred to as an Anger-type camera. The first of these cameras is described in U.S. Pat. No. 3,011,057 issued Nov. 28, 1961, to Hal O. Anger and entitled "Radiation Imaging Device".
An Anger camera uses a disc-shaped, thallium activated sodium iodide crystal. When a gamma ray strikes the crystal, a light scintillation occurs. Since sodium iodide is hygroscopic it is encased in a hermetically sealed envelope. The envelope includes an output window of glass or other clear material to permit "viewing" of the scintillations.
The Anger patent shows an hexagonal array of seven phototubes which are optically coupled to the output window by a light pipe. The phototubes have flat input windows which are disposed in a common plane paralleling the crystal. The phototube windows are optically coupled to the light pipe to provide efficient transmission of light from the crystal through the window and the light pipe to the phototubes. The phototubes emit electrical signals each of which is proportional to the amount of incident light received from a scintillation by a phototube.
The camera of the Anger patent has circuitry which processes the outputs of the phototubes to produce X and Y co-ordinate signals and a Z signal which is the sum of the tube outputs for a given scintillation. The co-ordinate signals are supplied to deflector plates of an oscilloscope which is unblanked when the Z signal is of an appropriate value. These co-ordinate signals will produce a spot of light on an output phosphor of the oscilloscope at a location corresponding to the locus of the scintillation that produced the signals.
As larger scintillation crystals became available, Anger-type cameras were produced with larger hexagonal phototube arrays. Indeed, all commercial cameras have had more than seven tubes. For a number of years, commercial cameras were made with nineteen tubes. Now thirty-seven tube cameras have supplanted the nineteen-tube cameras as the most popular and sixty-one tube cameras are being produced.
The resolving capability of an Anger-type camera is at least in part a function of the spacing between the crystal and the plane of the phototube input windows. Martone et al U.S. Pat. No. 3,784,819 issued Jan. 8, 1974, entitled "Scintillation Camera with Light Diffusion System" discloses and claims a masking technique which has enjoyed great commercial success. An objective of the Martone patent disclosure is to permit phototubes to be moved closer to the crystal to improve resolution while at the same time providing a camera which has outstanding uniformity and linearity characteristics. Electronic techniques have also been developed for compensating for losses of uniformity and linearity that would otherwise occur if phototubes are positioned close to the crystal for high resolution. These techniques have permitted the use of thinner light pipes and output windows, which are in fact also light pipes. In fact, one manufacturer uses the output window of the crystal envelope as the total light pipe without an interposed member between the window and the phototubes.
Not only have light pipes become thinner as camera technology has progressed, but also the crystals themselves have become thinner. Thinner crystals have come into use due in large part to the development of lower energy isotopes a reasonable percentage of which will scintillate in, rather than penetrate through, a thin crystal.
As the crystals, light pipes and windows have become thinner and the numbers of tubes and the crystal diameters have increased, a problem has developed. The problem is maintaining the phototubes in the desired optically coupled relationship with the light pipe without excessive pressures biasing the tubes against the light pipe to maintain optical coupling. With the thin optical components and their large diameters, biasing of phototubes against the light pipe has resulted in excessive crystal cracking, especially during manufacturing operations.
Since a cracked crystal is useless, and a camera manufacturer's cost for a crystal assembly is of the order of $3,000, that there is a problem should be apparent. Further, crystal cracking often occurs at a relatively late stage in detector assembly, resulting in considerable lost assembly time and time to correct the problem.